Frozen ground and particularly permafrost as well as hard rock which can be trenched by mechanical cutting possess very high resistance to digging and exhibit abrasive properties. In trenching, the natural properties of such materials are the cause of appreciable power consumption and high maintenance costs.
Besides, the resistance of frozen ground to digging, varying with trench depth, results in a high dynamic loading during the process of trenching and an attendant low reliability for continuous trenching machines.
Local irregularities on the surface of frozen ground cannot be levelled, and therefore the construction of a trenching machine should provide a means so that its travel and operation along such routes is enabled.
The requirements to reduce power consumption and maintenance costs, to improve the reliability of trenching machines, to provide for their operation on a rough ground, as well as other requirements made it necessary to develop trenching machines whose construction and operation are based upon a variety of principles.
In the general practice of cutting slits and trenches in frozen ground extensive use has been made of the designs peculiar to cutters of coal-cutting and coal-mining machines. Thus, for example, there known in the prior art a ladder trenching machine T -161 with a replaceable trenching part for cutting frozen ground, taken from a coal-cutting machine (Cf. "Mashiny dlya razrabotki merzlykh gruntov" /Machines for Cutting Frozen Ground/ edited by V. D. Telushkin, Mashinostroenie Publishing Company, Moscow, 1973, pp. 146-150), a three-jib trenching machine for cutting frozen ground (Cf. Stroietelnye i dorozhnye mashiny, 1978, No. 5, Mashinostroenie Publishing Company, pp. 4-6), and Roc Saw ladder trenching machine developed by Boring and Tunneling Company of America, Houston, Tex. (Cf. Highway and Heavy Construction, July 1976, p. 62, and May 1978, pp. 122-123).
These trenching machines comprise a vehicle of a tractor type, a digging apparatus, a spoil discharge conveyor assembly, and a drive mechanism. The digging apparatus comprises a boom and a support and guide slide means with an endless cutting chain around it. The cutting chain consists of pivotally interconnected links having therein holes to receive teeth which are arranged in a fanshaped pattern (a so-called "herringbone" arrangement). A belt conveyor can be used as the spoil discharge conveyor assembly in the trenching machines of said type.
The trenching machines with such a digging apparatus are sufficiently efficient when cutting only narrow trenches (slits) in frozen ground and rock. As a result, when cutting trenches for pipelines and other line structures, the trenching machines with such a digging apparatus only prepare a front for other machines, primarily power shovels, cutting along the trench contour longitudinal and, if required, transverse slits. The use of cutters of coal-cutting machines for direct digging of trenches of a required section in frozen ground and rock is much less efficient since the teeth mounted on such cutters in a fan-shaped arrangement fill up the entire trench section and pulverize these highly firm and abrasive materials to a great extent. This results in a high specific power consumption and a substantial consumption of the teeth with expensive carbide tips. When it is required to cut wide trenches, a digging apparatus, as a rule, is completed with an appropriate number of such cutting chains mounted in parallel. Such a design not only complicates the construction of the digging apparatus and raises its cost, but still further increases specific power consumption and the consumption of the teeth.
The endless cutting chain in this construction of the trenching machine moves along the entire length of the support and guide slide means. Such a design provides a reliable support and guiding of the cutting chain, but requires a high power consumption to overcome sliding friction and causes a rapid wear of the support and guide means and of the cutting chain itself.
The use of a belt conveyor as the spoil discharge conveyor assembly requires that the distance between the vehicle and the digging apparatus should be enlarged to accomodate this rather wide conveyor. The belt conveyor is mounted at such a height which provides free travel over irregularities of the ground surface. Besides, combination of this digging apparatus with the belt conveyor causes the increase of the required height of spoil discharge. Thus, the use of the belt conveyor results in the increase of the mass and size of the trenching machine.
Also known in the art is a ladder trenching machine Type T -208A specially designed to cut frozen ground (Cf. an article "Exkavator transheyny tsepnoy T -208A / T -208A Ladder Trenching Machine/ by S. Kh. Vartanov et al. published in the section "Digging Machines" of a journal "Stroitelnye i dorozhnye mashiny", 1978, No. 12, pp. 4-6), which is the nearest in its principle to the proposed invention and chosen as the prototype.
Said trenching machine comprises a vehicle of a tractor type, a digging apparatus, a spoil discharge conveyor assembly, and a drive mechanism. The digging apparatus comprises a boom connected with the vehicle, a support and guide rolling means mounted on the boom, and an endless draft chain around said means, said chain being made as a tractor caterpillar whereto carrying members with teeth and wedges are attached by threaded fastening elements. The teeth and wedges are mounted along longitudinal lines, and the teeth are higher than the wedges. The teeth loosen successively and continuously frozen ground mass by cuttings, and ground blocks left therebetween are periodically spalled due to a side thrust of the wedges. The support and guide means comprise a wheel mounted at the free end of the boom, a plurality of rollers arranged along said boom, and a take up means. The spool discharge conveyor assembly is made as a flight conveyor comprising a frame with arms mounted thereon and carrying drive and idler wheels, and an endless chain with flights which rounds these wheels. One of the arms on a side leg of the frame is pivoted to the frame and connected with a hydraulic cylinder to shift the side portion of the flight chain into travelling position. The drive mechanism comprises a drive sprocket wheel disposed on the longitudinal axis of the digging apparatus and coupled through a transmission to the engine of the vehicle, and safety means.
The foregoing trenching machine is much more efficient when cutting trenches in frozen ground as compared to the trenching machines with a cutter similar to that of coal-cutting machines since it provides for the digging of a trench of a required section in one cut.
However, this trenching machine is insufficiently versatile in its application in frozen grounds of various structures, and its efficiency and serviceability do not fully meet arduous operating conditions.
The wedges of such a digging apparatus are rather efficient when spalling the blocks of hard-frozen ground exhibiting pronounced brittle properties. However, in initial entry and further cutting into frozen ground with predominant plastic properties the wedges do not spall the blocks of frozen ground due to their side thrust, but squash them, which reduces the output and does not provide versatile application of the trenching machine in frozen grounds of various structures. Besides, periodic action of the wedges results in an increased dynamics of the process and affects the reliability of the digging apparatus and of the entire trenching machine. Mounting of the teeth and wedges which are different in their construction on the same digging apparatus additionally complicates its fabrication and maintenance. The digging apparatus is also characterized in that the total number of the teeth and wedges mounted thereon still remains rather great.
Making the digging apparatus with an endless chain of a caterpillar type most fully meet the requirements of fabrication and conditions of operation of the digging apparatus of the trenching machines for cutting frozen ground. Such a chain is of sufficiently high strength and wear resistance. Besides, such chains are widely used in industry and their cost is rather low. However, connection of the members carrying the teeth with the chain of a caterpillar type by threaded fastening elements exhibits strength considerably lower than the breaking strength of the chain. This feature makes it impossible to employ to a full degree the strength properties of the endless chain of a caterpillar type. What is more, in the course of operation of this trenching machine ground pressing-on and freezing-on as well as jamming of stones between the chain links and the drive sprocket wheel of the drive mechanism occur which not only affects the efficiency of the trenching machine, but also may result in its failure. Takeup and safety means incorporated into the digging apparatus cannot prevent such failures. Another constructional feature of this trenching machine consists in the fact that the distance between the axis of the wheel mounted at the lower end of the boom and the axis of the nearest roller cannot be less than the pitch of the draft chain. Hence, with initial entry of the digging apparatus into frozen ground and when cutting trenches of a small depth, the draft chain is turned, the cutting angle of the teeth is consequently enlarged, and the dynamics of the process is increased.
The trenching machine of this construction is also characterized in that the position of the lower portion of the flight conveyor is not adjustable for height, which affects the performance of the trenching machine and the reliability of the conveyor when travelling and operating over rough ground.